U.S. Pat. No. 5,678,138 discloses a device for controlling sheets in a printing machine comprising means, which are provided to detect the arrival of at least one sheet, preferably each sheet, in a transport path at least once at least one marked location of said transport path, wherein an arrangement for generating at least one signal having a countable pulse, whereby a freely selectable but then fixed start of these pulses is assigned to the position of at least one frame of frames into which the minimum of one transport path segment is divided—relative to said marked location, and whereby the minimum of one sheet is controlled with the use of this pulse at least one other marked location.
U.S. Pat. No. 6,343,202 discloses another imaging forming apparatus, wherein the first marked location corresponds to the location of the sensor and the other locations correspond the transfer positions of image.
DE-A-102 34 629 explains, referring to an electrophotographically operating printing machine—which will be described in detail for the sake of clarity, without, however, representing a restriction to such a type of digital printing machine—how sheets to be printed are fed by one or more feeders to a paper path or, in more general terms, to a transport path for printing material of any type. Several feeders or feeding units provide printing material—specifically of different formats, weights, materials or the like. This is of particular advantage, especially in the case of a digital printing machine, because a new image is created anyhow for each new page to be printed, and thus even mixed print jobs can be processed without problems, namely those, in which, for example, such an individual print job consists of pages of a brochure which are directly fed in successive order to the printing unit and, subsequently, optionally also to a finishing step, in which case, for example, the front and rear cover sheets may consist of a heavier-weight paper and the subsequent papers may consist of a lighter-weight paper and, in between, even films of plastic material with diagrams or the like may be printed. Such different printing materials are made available in different feeding units and are fed in a pre-selected order to the transport path.
A first transport path segment that starts at the feeding units may consist, for example, of rotating driven grip belts, between which sheets are transported. Thereafter, the sheets could be transferred to and placed onto a rotating driven transport belt and adhere there due to electrostatic forces. In most cases, this transport belt is a transparent web of plastic material and passes through a printing system, which, for color printing, may of course comprise several printing units. In electrophotographic printing, one latent toner image per color separation is transferred to the sheet. Thereafter, the sheet is transported to a fusing unit, in which the toner image is fused to the printing material, specifically melt-deposited there, and cooled. Considering the transport into and through the fixing unit, a change of the transport member could again occur. Only sheets to be printed on one side are then continued to be transported or ejected into a tray. After the fusing step, sheets to be printed on both sides are returned to pass the printing unit and are turned over via a transport path loop for continued printing. The reverse transport and the turn may take place at the same time, for example, in that, also on this transport path segment, grip belts are used which take an approximately helical course and, in so doing, rotate the sheets about their longitudinal axes by 180 degrees.
In particular, the transport belt passing through the printing system, said belt being frequently referred to as a web in electrophotography, is to be loaded with sheets to be printed, whereby the space between sheets is to be small enough to achieve the greatest possible throughput per unit of time, i.e., to ensure the highest possible printing output. On the other hand, minimum distances between successive sheets must be maintained. This applies to simplex-printing of only the front side of sheets, as well as to duplex-printing when the front and rear sides of the sheets are printed and perfected.
In order to achieve an optimal or matched loading of the web, the web is divided virtually, or also by means of controllers, into areas which can be described as frames, in which respectively one sheet—taking into consideration common formats—is to be precisely placed for printing. In so doing, an area of the web is recessed, said area optionally having a transverse seam, by means of which the ends of the web are connected in order to form a closed loop. Usually, for convenience, this seam is also used as a mark that is detected by a sensor in order to allow a control of the rotary position of the web and to have a reference point. Therefore, this seam must not be covered by a sheet. Other marks could also be taken into consideration, in particular those which are applied only along the edge of the web.
To ensure, even in duplex-printing mode, that these frames on the transport belt will be exactly met again after the return transport of the sheets in order for the sheets to be transferred, the ratio of the running time of the sheets rotating via the return after the first side has been printed with respect to the running time of the web must represent a whole number.
However, despite this, problems may still occur in that sheet running times inside the considered printing machine are affected by various parameters. For example, the weight of the paper and the length of the paper have been found to represent dominant paper variables. Likewise, machine-specific parameters such as, for example, exact transport path length, roller diameter and motor speeds are contributing factors.
This behavior is the reason for various problems (e.g., image quality, insufficient distance between paper sheets) that occur when the machine is running. This is particularly noticeable when mixed paper print jobs are run. For example, thick (heavy) sheets have shorter running times than thin sheets. Consequently, during their run through the machine, the distance between two successive sheets can decrease distinctly (the fast, thick one catches up), this leading to an interruption of the printing function due to too small a distance between the sheets and hence to a clear loss of machine performance. Likewise, sheets could be deposited on the web seam and thus trigger image errors.
Therefore, the cited DE-A-102 34 629 suggests that a starting point for feeding a sheet from any, or the only, feeding unit is selected with respect to the type of printing material of which said sheet consists.
In this reference, advantageously, sheets are started depending on the type—specifically their length and/or weight—at different times, i.e., fed by the respective feeding unit to the transport path, in order to apply a counter-error to potentially (even with respect to each other) skewed sheets as expected during transport for correction at the onset, so that the desired position will be taken during transport. In order to be able to perform such a preliminary control in a quantitatively targeted manner, a development in this case provides that information for the selection of the starting time is yielded beforehand by at least one trial run with at least one type of printing material, preferably by trial runs with different types of material, while a corresponding empirical table is created, for example, configured as a look-up table, i.e., a specified table.
However, it has been found that the generation and use of such tables is very complex and still does not always provide fully satisfactory results.
The object of the invention is to show a possibility of improving the control of the sheets themselves during the printing operation.